FIELD OF THE INVENTION
This invention relates generally to Atomic Force Microscopes (AFM) and more particularly to means for measuring the forces and/or deflections occurring during operation of the AFM. The measurement is performed by means of a pointed tip attached to an oscillating quartz crystal.
The Atomic Force Microscope proposed by G. Binnig (EP Appln. No. 86 110 276.2) and described by G. Binnig, C. F. Quate and Ch. Gerber, Phys. Rev. Letters, Vol. 56, No. 9, March 1986, pp. 930-933, employs a sharply pointed tip attached to a spring-like cantilever beam to scan the profile of a surface to be investigated. At the distances involved, attractive or repulsive forces occur between the atoms at the apex of the tip and those at the surface, resulting in tinyl deflections of the cantilever beam. In Binnig's proposal, this deflection is measured by means of a tunneling microscope, i.e., an electrically conductive tunnel tip is placed within tunnel distance from the back of the cantilever beam, and the variations of the tunneling current are used to measure the deflection. With known characteristics of the cantilever beam, the forces occurring between the AFM tip and the surface under investigation can be determined.
The forces occurring between a pointed tip and a surface are usually described as vander-Waals forces, covalent forces, ionic forces, or repulsive core interaction forces. The energies involved in the atomic distance approach of a single atom (at the apex of the tip) to a surface are the range of E.sub.0 =0.01 . . . 10 eV=10.sup.-22 . . . 10.sup.-18 Joule. The corresponding distances are in the subnanometer range of x.sub.0 =10 . . . 1000.+-.=0.01 . . . 1 nm. The respective forces, i.e., the first derivatives of the potential function, therefore, are in the range of K.sub.0 =10 nN . . . 10 nN. The resulting atomic "spring constants`, i.e., the second derivatives of the potential function are in the range of C.sub.0 =100 . . . 0.01 N/m. These data can be deduced from surface studies and many other sources, such as the values of elastic constants.
It is one object of the present invention to describe a force measuring device which may be used as an atomic force microscope, but which does not employ cantilever beams nor tunneling for detection.
Accordingly, the present invention proposes an atomic force microcope comprising a pointed tip provided for interaction with a surface to be investigated and means for approaching said tip to said surface to within a working distance on the order of one tenth of a nanometer, and for scanning said tip across said surface in a matrix fashion. This atomic force microscope is characterized in that said tip is attached to one surface of an oscillating body carrying, on opposite sides thereof, a pair of electrodes permitting an electrical potential to be applied, that, in operation and with said tip remote from said surface, said body is excited to oscillate at its resonance frequency, and that, with said tip maintained at said working distance from said surface, said body osillates at a frequency deviating in a characteristic manner from said resonance frequency, that said deviation is compared with a reference signal, and that the resulting differential signal is passed through a feedback loop to control said means for approaching the tip to said surface.